The reaction of olefins with C-5 mercurated pyrimidine nucleosides and lithium palladium chloride as a route to C-5 substituted pyrimidine nucleosides is under investigation. Among the C-5 substituted pyrimidine nucleosides are antiviral agents, fluorescent nucleoside analogs, irreversible enzyme inhibitors, and intermediates for the synthesis of more complex heterocyclic systems.